SNMP Transport: SNMP Communications Services

Network Working Group                              F. Kastenholz, Editor
Request for Comments: 1270               Clearpoint Research Corporation
                                                            October 1991


                      SNMP Communications Services

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard.  Distribution of this memo is
   unlimited.

Table of Contents

   1. Abstract ..............................................    1
   2. Introduction ..........................................    1
   3. Standardization .......................................    3
   4. Interoperability ......................................    3
   5. To Transport or Not To Transport ......................    3
   6. Connection Oriented vs. Connectionless ................    6
   7. Which Protocol ........................................    8
   8. Security Considerations ...............................    9
   9. Appendix ..............................................    9
   10. References ...........................................   10
   11. Acknowledgements .....................................   11
   12. Author's Address .....................................   11

1.  Abstract

   This memo is being distributed to members of the Internet community as
   an Informational RFC.  The intent is to present a discussion on the
   issues relating to the communications services for SNMP.  While the
   issues discussed may not be directly relevant to the research problems
   of the Internet, they may be interesting to a number of researchers
   and implementors.

2.  Introduction

   This document discusses various issues to be considered when
   determining the underlying communications services to be used by an
   SNMP implementation.

   As reported in RFC 1052, IAB Recommendations for the Development of
   Internet Network Management Standards [8], a two-prong strategy for
   network management of TCP/IP-based internets was undertaken.  In the
   short-term, the Simple Network Management Protocol (SNMP), defined in
   RFC 1067, was to be used to manage nodes in the Internet community.

   In the long-term, the use of the OSI network management framework was
   to be examined.  Two documents were produced to define the management
   information: RFC 1065, which defined the Structure of Management
   Information (SMI), and RFC 1066, which defined the Management
   Information Base (MIB).  Both of these documents were designed so as
   to be compatible with both the SNMP and the OSI network management
   framework.

   This strategy was quite successful in the short-term: Internet-based
   network management technology was fielded, by both the research and
   commercial communities, within a few months.  As a result of this,
   portions of the Internet community became network manageable in a
   timely fashion.

   In May of 1990, the core documents were elevated to "Standard
   Protocols" with "Recommended" status.  As such, the Internet-standard
   network management framework consists of: Structure and Identification
   of Management Information for TCP/IP-based internets, RFC 1155 [9],
   which describes how managed objects contained in the MIB are defined;
   Management Information Base for Network Management of TCP/IP-based
   internets, which describes the managed objects contained in the MIB,
   RFC 1156 [10]; and, the Simple Network Management Protocol, RFC 1157
   [1], which defines the protocol used to manage these objects.

   In parallel with this activity, documents specifying how to transport
   SNMP messages over protocols other than UDP/IP have been developed and
   published: SNMP Over Ethernet [3], SNMP Over OSI [2], and SNMP Over
   IPX [6] and it would be suprising if more were not developed.  These
   memos have caused a degree of confusion in the community.  This
   document is intended to disperse that confusion by discussing the
   issues of relevance to an implementor when choosing how to encapsulate
   SNMP packets.

   None of these documents have been made full Internet Standards. SNMP
   Over ISO and SNMP Over Ethernet are both Experimental protocols. SNMP
   Over IPX [6] is an Internet Draft. Only the SNMP Specification [1] is
   an Internet Standard.

   No single transport scheme can be considered the absolute best
   solution for all circumstances.  This note will argue that, except for
   a very small set of special circumstances, operating SNMP over UDP/IP
   is the optimal scheme.

   This document does not present a standard or a protocol for the
   Internet Community.  For production use in the Internet the SNMP and
   its required communication services are specified in [1].

3.  Standardization

   Currently, the SNMP Specification [1] only specifies that the UDP
   protocol be used to exchange SNMP messages.  While the IAB may
   standardize other protocols for use in exchanging SNMP messages in the
   future, only UDP is currently standardized for this purpose.

   In order to claim full compliance with the SNMP Specification, an
   implementation would have to use UDP for SNMP message exchange.

4.  Interoperability

   Interoperability is the degree to which the equipment produced by one
   vendor can can operate with equipment produced by another vendor.

   Related to Interoperability is compliance with a standard.  Everything
   else being equal, a device that complies with some standard is more
   likely to be interoperable than a device that does not.

   For commercial product development, the pros and cons of developing an
   interoperable product must be weighed and a choice made.  Both
   engineering and marketing organizations participate in this process.

   The Internet is the single largest market for SNMP systems.  A large
   portion of SNMP systems will be developed with the Internet as a
   target environment.  Therefore, it may be expected that the Internet's
   needs and requirements will be the driving force for SNMP.  SNMP over
   UDP/IP is specified as the "Internet Standard" protocol.  Therefore,
   in order to operate in the Internet and be managed in that environment
   on a production basis, a device must support SNMP over UDP/IP.  This
   situation will lead to SNMP over UDP/IP being the most common method
   of operating SNMP.  Therefore, the widest degree of interoperability
   and widest acceptance of a commercial product will be attained by
   operating SNMP over UDP/IP.

   The preponderance of UDP/IP based network management stations also
   strongly suggests that an agent should operate SNMP over UDP/IP.

   The results of the interoperability decision drive a number of
   technical decisions.  If interoperability is desired, then SNMP must be
   operated over UDP/IP.

5.  To Transport or Not To Transport

   A major issue is whether SNMP should run on top of a transport-layer
   protocol (such as UDP) or not.  Typically, the choice is to run over a
   transport/network/data link protocol or just run over the datalink.
   In fact, several protocols have been published for operating SNMP over
   several different datalink and transport protocols.

   Operation of SNMP over a Transport and Network protocol stack
   is preferred.  These protocols provide at least five functions
   that are of vital importance to the movement of SNMP packets
   through a network:

          o Routing
               The network layer provides routing functions, which
               improves the overall utility of network management.  The
               network has the ability to re-route packets around failed
               areas.  This allows network management to continue
               operating during localized losses of service (It should
               be noted that these losses of service occur not only
               because of failures, but also for non-failure reasons
               such as preventive maintenance).

          o Media Independence
               The network layer provides a high degree of media
               independence.  By using this capability, many different
               types of network elements may be managed.  Tying SNMP to
               a particular data link protocol limits the management
               scope of those SNMP entities to just those devices that
               use that datalink protocol.

          o End-to-End Checksum
               The end-to-end checksum provided by transport protocols
               improves the reliability of the data transfer.

          o Multiplexing/Demultiplexing
               Transport protocols provide multiplexing and
               demultiplexing services.  These services facilitate the
               many-to-many management relationships possible with SNMP.

          o Fragmentation and Reassembly
               This is related to media independence.  IP allows SNMP
               packets to transit media with differing MTU sizes.  This
               capability is not available for datalink specific
               transmission schemes.

               Fragmentation and Reassembly does reduce the overall
               robustness of network management since, if any single
               fragment is lost along the way, the operation will fail.
               The worse the network operates, the higher the
               probability that a fragment will get lost or delayed.
               For monitoring and data gathering while the network is
               operating normally, Fragmentation and Reassembly is not a
               problem. When the network is operating poorly (and the
               network operators are typically trying to diagnose and
               repair a failure), small packets should to be used,
               preventing the packet from being fragmented.

   There are other services and functions that are provided by a
   connection oriented transport.  These services and functions are not
   desired for SNMP.  A later section will explore this issue in more
   detail.

   The main drawbacks that are cited with respect to using Transport and
   Network layers in the managed object are: a) Increased development
   time and b) Increased resource requirements.  These arguments are
   less than compelling.

   There are several excellent public domain or freely redistributable
   UDP/IP stacks that provide enough support for SNMP.  The effort
   required to port the essential components of one of these stacks is
   small compared to the overall effort of installing the SNMP software.

   The additional resources required in the managed object to support
   UDP/IP are minimal.  CPU resources are required only when actually
   transmitting or receiving a packet.  The largest single resource
   requirement of a UDP/IP is calculating the UDP checksum, which is
   very small compared to the cost of doing the ASN.1 encoding/decoding,
   Object Identifier lookup, and so on.

   The author has personal knowledge of a UDP/IP stack that was
   developed expressly for the purpose of supporting SNMP.  This stack
   requires less than 4Kb of code space.  It is a minimalist
   implementation of UDP/IP in that it is "just enough" so support SNMP.
   This stack supports UDP, IP, ARP, and handles ICMP redirect and echo
   request messages.  Furthermore, this stack was developed by a single
   person in approximately two months.  Obviously, neither the
   development effort nor the memory requirements are large.

   The network overhead of using UDP/IP is relatively small.  A UDP/IP
   header requires 28 octets (assuming no IP options).  Since the UDP is
   connectionless, it will generate no overhead traffic of its own (such
   as TCP SYNs, FINs, and ACKs).

   The growing popularity of internetworking outside of The Internet
   mandates that SNMP operate over, at least, a network layer protocol.
   These internetworks consist of a number of networks all connected
   together with routers.  In order to traverse a router, a packet must
   be one of the network layer protocols that the router understands.
   Therefore, for SNMP management to be deployed in an internetwork, the
   SNMP entities in that internetwork must use a network layer protocol.
   SNMP over a datalink can not traverse a router.

   There are some circumstances where running SNMP over some datalink is
   appropriate.

   There are schemes are under development to provide Out-Of-Band (OOB)
   management access to network devices.  This OOB access is typically
   provided over point-to-point or dial-up connections.  Since these
   connections are dedicated to OOB network management and go directly
   from the network management station to the managed device, a
   Transport/Network protocol may not be necessary.

   Using a Transport/Network protocol on these links may be easier from
   a development point of view though.  It is probably a simple
   configuration operation to have the management station's IP use a
   serial port rather than the "normal" (e.g., Ethernet) port for
   traffic destined for a particular node.

   If the Out-Of-Band link is also used as a "primary" route to some
   nodes, then the functions of a network-layer are required.  These
   functions are readily supplied by using UDP/IP.

   For a datalink interface and driver (e.g., a PC Ethernet interface
   card) that must be manageable independent of the higher level
   protocol suite (which might NOT be manageable), operating SNMP
   directly over the datalink is reasonable.  It is not known, a priori,
   what higher-level protocol services may be available, so those
   services can not be used.  If an arbitrary choice is made for
   example, to put in an elementary UDP/IP stack, then there may be two
   independent UDP/IPs in the system (which is undesireable as this
   would require two IP addresses per managed node), or a new protocol
   stack will be introduced into the environment.

6.  Connection Oriented vs. Connectionless

   While this section primarily addresses itself to transport layer
   issues, its basic discussion of connection oriented vs connectionless
   applies to any layer which provides communication services for SNMP.

   For SNMP, connectionless transport service (UDP) is specified in the
   Protocol Specification [1].  This choice was made after careful study
   and consideration by the original SNMP developers.

   The prime motivation of this choice is that SNMP must continue to
   operate (if at all possible) when the network is operating at its
   worst.  For other applications, such as Telnet or FTP, the user can
   always "try again later" if the network is operating poorly.  On the
   other hand, the major purpose of a network management protocol is to
   fix the network when it is operating poorly so the "try again later"
   strategy is useless.

   By using a connectionless transport protocol, SNMP takes on the
   responsibility of reliable data transmission (A SNMP application may
   time out outstanding requests and either retransmit them or abort
   them as appropriate).  However, the SNMP requires these functions
   only of the sender of a Request PDU (get, getNext, or Set), which
   typically is a network management station.  Since the Agent only
   generates responses, it need not perform any of these functions.
   This vastly reduces the resource and functional requirements on the
   Agent.

   If a connection oriented transport is used, then a fundamental design
   choice must be made with respect to connection maintenance:

          (1)  Keep a connection open to each managed object on the
               network,

          (2)  Establish and tear down connections on a per-operation
               basis, or

          (3)  Keep a fixed number of connections open and, when another
               connection is needed, use some algorithm (e.g., LRU) to
               select one for closing and opening to the new agent.

   All of these alternatives pose severe problems, and because of them,
   each is undesirable.

   The first option reduces the amount of resources required to perform
   a single operation in that the connection establishment and
   termination cost is "amortized" over many operations.  On the other
   hand, keeping a connection open implies that the management station
   needs to maintain a large number of connection records (in the
   hundreds or even thousands).  Furthermore, if either side of the
   connection engages in "keep-alives" (even though such behavior is
   frowned upon), a large amount of traffic will be generated, consuming
   a large amount of network resources, all for no gain.

   The second option reduces the amount of idle resources such as
   connection records, but vastly increases the amount of resources
   required to perform an operation.  A connection must be established,
   the request Message sent and the response returned, and then the
   connection closed for each operation.  For a TCP, this would
   typically require 10 separate packet transfers plus the TCP Time-Wait
   (see the Appendix for details).

   In the face of pathological network problems, a connection oriented
   transport protocol may simply cease to operate because the
   probability of getting all of these packets through reduces to a very
   small number.

   The third option requires that the management station maintain
   connection usage information in order to implement the LRU algorithm.
   This excessively complicates the management station.  Furthermore,
   this option tends to reduce to the second option when doing health
   check polling for a number of agents that is large compared to the
   number of supported connections.

   A connection oriented transport protocol may provide services that
   are undesirable or unneeded by SNMP.

   For example, one application of network management is to poll nodes
   to determine if they are up or not.  When a node is up, it makes
   little difference whether SNMP operates over TCP or UDP.  However, if
   the node goes down then TCP will eventually close the connection.
   Every poll request must then be translated into a TCP Open request
   while the managed node is down.  Once the node comes up, the send
   must then be done.

   For connection oriented transports, the transport ACK does not
   necessarily indicate delivery of data to the destination application
   process (for TCP, see section 2.6 of [4]).  The SNMP would still need
   its own timeout/retry procedure to ensure that the SNMP software
   actually got the packet.

   A connection oriented transport such as TCP provides flow control for
   the data stream.  Because of the lock-step nature of SNMP protocol
   exchanges, this is not a service that SNMP requires.

   Architectural purists may argue that an "Application" layer entity
   (SNMP) must not perform operations that are properly the realm of the
   Transport layer (timeouts, retransmissions, and so on).  While
   architecturally pure, this line of reasoning is not relevant.  The
   network management applications and protocols are monitoring the
   "health" of the network and, as a result, have the best information
   and are in the best position to adapt their own behavior to the state
   of the network, and thereby, continuing operations in the face of
   network adversity.

7.  Which Protocol

   The final point of discussion is the actual choice of a protocol to
   support SNMP.

   If a device is destined for use in the Internet then it must operate
   SNMP over UDP/IP.

   If the device is operating in some other protocol environment, then
   SNMP ought to use the transport services that are native to that
   environment.  It may make very little sense to introduce a new
   protocol stack into a network in order to provide just one service.
   For example, it could require that the network operations staff
   understand and be able to administer and operate two protocol stacks,
   that hosts and routers understand both protocols, and so on.  It may
   also be bureaucratically impossible to introduce UDP/IP into the
   environment (the "We are only a FOONET shop - if it doesn't speak
   FOONET, we don't want it" argument).

   References [2] and [6] are experimental standards for operating SNMP
   over IPX and OSI respectively.  In these environments, those
   standards ought to be adhered to.

8.  Security Considerations

   Security issues are not discussed in this memo.

9.  Appendix

   This appendix details the TCP packet transfers required to perform a
   single SNMP operation assuming that the connection is established
   only for that operation and that a single SNMP operation (e.g., get
   request) is performed.  We also assume that all operations are
   "normal" i.e., that there are no lost packets, no simultaneous opens,
   no half opens, and no simultaneous closes.  We also ignore the
   possibility of TCP segmentation and IP fragmentation.

   The nomenclature used to illustrate the packet transactions is the
   same as that used in the TCP Specification [4].

              Packet  Management                         Managed
              Number  Station                            Object
                               Connection Open...
               1         >--<CTL=SYN>----------------------->
               2         <--<CTL=SYN,ACK>-------------------<
               3         >--<CTL=ACK>----------------------->
                           Connection now open,
                           SNMP Request is sent.
               4         >--<DATA=SNMP Request>------------->
                           Response comes back
               5         <--<DATA=SNMP Response, CTL=ACK>---<
               6         >--<CTL=ACK>----------------------->
                           Operation is complete,
                           Management station initiates the
                           close.
               7         >--<CTL=FIN,ACK>------------------->
               8         <--<CTL=ACK>-----------------------<
               9         <--<CTL=FIN,ACK>-------------------<
              10         >--<CTL=ACK>----------------------->
                          Wait 2 MSL
                          Connection now closed.

   Some optimizations are possible IF the TCP has knowledge of the type
   of operation.  However, a general purpose TCP would not be tuned to
   SNMP operations so those optimizations would not be done.

10.  References

   [1] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple
       Network Management Protocol", RFC 1157, SNMP Research,
       Performance Systems International, Performance Systems
       International, MIT Laboratory for Computer Science, May 1990.

   [2] Rose, M., Editor, "SNMP over OSI", RFC 1161, Performance Systems
       International, Inc., June 1990.

   [3] Schoffstall, M., Davin, C., Fedor, M., and J. Case, "SNMP over
       Ethernet", RFC 1089, Rensselaer Polytechnic Institute, MIT
       Laboratory for Computer Science, NYSERNet, Inc., University of
       Tennessee at Knoxville, February 1989.

   [4] Postel, J., "Transmission Control Protocol - DARPA Internet
       Program Protocol Specification", RFC 793, DARPA, September 1981.

   [5] Postel, J., "User Datagram Protocol", RFC 768, USC/Information
       Sciences Institute, August 1980.

   [6] Wormley, R., "SNMP Over IPX", draft in process, August 1990.

   [7] Postel, J., Editor, "IAB Official Protocol Standards", RFC 1250,
       IAB, August 1991.

   [8] Cerf, V., "IAB Recommendations for the Development of Internet
       Network Management Standards", RFC 1052, NRI, April 1988.

   [9] Rose M., and K. McCloghrie, "Structure and Identification of
       Management Information for TCP/IP-based internets", RFC 1155,
       Performance Systems International, Hughes LAN Systems, May 1990.

  [10] McCloghrie K., and M. Rose, "Management Information Base for
       Network Management of TCP/IP-based internets", RFC 1156, Hughes
       LAN Systems, Performance Systems International, May 1990.

11.  Acknowledgements

   The author wishes to thank Jeff Case, Chuck Davin and Keith
   McCloghrie for their technical and editorial contributions to this
   document.

12.  Author's Address

   Frank Kastenholz
   Clearpoint Research Corporation
   35 Parkwood Drive
   Hopkinton, Mass. 01748

   Phone: (508) 435-2000

   Email: kasten@europa.clearpoint.com